Hello all I'm new here. I have a 2008 Chevy 3500 HD with the Duramax diesel.
I bought the truck new and off the lot I was getting an average of 400 miles to a tank. I now have 46k miles on the truck and have taken it only to the dealer for service. Some time over the last few months could have been as long as six months my fuel mileage has dropped to 300 miles a tank. This change in mileage happened over night and was not a slow decrease. The truck sounds and runs as good as the day I took it off the lot.

I'm thinking that the dealer flashed my ECM when it went in for service but they claim they have no part in my loss of miles. They also tell me there is no going back to the original programming.

So what do you think, is this possible?

The mileage is bad enough I'm thinking of voiding my warranty and fixing the ECM. I'm figuring that if I got the 400 miles per tank back my fuel savings could buy a new motor if anything went wrong. I've also had bad luck having warranties honored in the past. So I don't know.

Do you have larger than stock tires? Tires inflated properly?
Have you cleaned the MAF sensor? ---changed the air filter?
You have the DPF? If so it is possible that it is regenerating a lot, and that uses fuel.

Do you have a long bed-short bed- ? 26 gallon on the short, and 34 on the large.

Thanks for the interest in my problem.
I'm near Christiansburg VA.
As for the truck. It runs and sounds great and pulls the hills around here great even with a trailer loaded so nothing has changed there.
The truck is all stock the only thing new to the truck is a set of front tires and they are the same as the ones that wore out.

The truck is a long bed with crew cab and fuel filters and air filters were taken care at service. I don't know what a DPF is or the MAF sensor is. The tank is the 26 gallon.

I'm taking it to a different dealer next Tuesday. I will give them a shot at fixing my problem.

About a year ago the dealer offered to flash my ECM to accept the northern fuels that I guess is the ethanol diesel. I declined because it said it would effect fuel mileage. I still have that paper around here somewhere with that information on it,

year, model, mods, even name if you want. kinda show whats done to the truck which in your case its stock. people just want to know what year of truck you are talking about and when it goes to a problem they want to know if you have any mods that could've made a difference

....I'm thinking that the dealer flashed my ECM when it went in for service but they claim they have no part in my loss of miles. They also tell me there is no going back to the original programming.

So what do you think, is this possible?

The mileage is bad enough I'm thinking of voiding my warranty and fixing the ECM. I'm figuring that if I got the 400 miles per tank back my fuel savings could buy a new motor if anything went wrong. I've also had bad luck having warranties honored in the past. So I don't know.

Any thoughts.

I got 18.6mpg on my old 08 LMM, took it the dealer for a recall (heated washer fluid). I got the truck back I never saw over 17.0 mpg. I'm pretty sure they flashed it and something changed in the program dramatically

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